To reproduce:

1. Download attached project "UnityPluginTest.zip" and open in Unity
2. In Project view observe that there is a "Test.plugin" folder and in it, there is "Test.cs" script
3. Build for iOS
4. Open folder where You built Xcode project and open "Libraries" folder in it
5. Find "Test.plugin" Plug-in file, right click it and press "Show Packages Content"

Expected results: Under Libraries/Test.plugin, there's "Test.cs" file
Actual results: Under Libraries/Test.plugin, there's no "Test.cs" file but a "Test.cs" folder

Notes:
- From Unity 2018.3.0a6 to Unity 2018.3.3f1 under Libraries folder there is no "Test.plugin" Plug-in file but only "Test.cs" file

Reproduced on Unity 2018.3.0a6, 2018.3.7f1, 2019.1.0b5 and 2019.2.0a6
Not reproduced on Unity 2018.2.20f1 and 2018.3.0a5
Regression on Unity 2018.3.0a6